Light Neutral Higgs Bosons at the Low Energy γγ Collider

A light neutral Higgs boson in the framework of the general two Higgs doublet model (2HDM) is not excluded by existing data. We point out that it can be looked for at the proposed low energy γγ collider. Failure to detect one may lead to important limits on the parameters of the general 2HDM. [email protected] [email protected] Supported in part by grants from Polish Committee for Scientific Research and the EC grant under the contract CHRX-CT92-0004. While the Standard Model (SM) Higgs scalar as well as the MSSM neutral Higgs particles have been constrained by LEP1 data to be heavier than 65.2 GeV, and 40–50 GeV, respectively, the general two Higgs doublet model (2HDM) may yet accomodate a very light ( < ∼ 40 GeV) neutral scalar h or a pseudoscalar A as long as Mh + MA > ∼ MZ [1, 2]. The interesting case of very light (∼ few GeV) Higgs particles has been studied in dedicated experiments, for example, in the Wilczek process [3]. Unfortunately, limits are not decisive, especially due to large theoretical uncertainties in QCD and in relativistic corrections. There is some hope though that better limits may be obtained by exploring the Yukawa process (Z → f f̄h/A) in the existing LEP1 data [4] or in improved (g − 2)μ measurements [5]. The γγ option at the Next Linear Collider, on the other hand, may provide an excelent opportunity to search for a very light neutral Higgs particle. We focus here on the resonant production of a very light neutral Higgs particle at the low energy γγ collider, suggested as a test machine for the NLC [6]. The general 2HDM is charactarized by five (Higgs) masses and two parameters (angles): α and β [7]. We consider here the phenomenologically appealing version, where the neutral components of the two doublets φ1,2 (with vacuum expectation values v1,2) couple exclusively to the I3 = ±1/2 fermion fields. Tree level flavour changing neutral currents, then, vanish identically. Such an assumption could lead naturally to a large value for the ratio tanβ ≡ v2/v1 (∼ mt/mb ≫ 1) and, thus, to an enhanced coupling of the light scalar (pseudoscalar) to the down-type quarks and the charged leptons, while suppressing the coupling to the up-type quarks. In addition to the above, the extension to the 2HDM results in significant modification in the scalar–vector boson sector of the theory. The canonical Higgs boson production mechanism, namely the Bjorken process Z → Zh, now proceeds with a rate proportional to sin(α − β). Negative results at LEP1 thus imply that sin(α − β) < 0.1 if Mh < ∼ 50 GeV. More than this, a strikingly new feature is that the Z can now couple to a pair of nonidentical spin-0 objects, leading to new Higgs particle production mechanisms. Of particular interest is the process Z → h + A, with a rate ∝ cos(α − β). A lack of such events at LEP1 can then be translated to a constraint in the three-dimensional (Mh,MA, α − β) parameter space [2]. For a given (Mh,MA) combine, this obviously translates to an relatively strong upper limit on cos2(α− β). The two constraints are thus complementary to each other. In addition, one has also to consider the fact that a non-trivial Higgs sector may lead to additional contribution to the Z width, even if the new decay channels cannot be identified over the SM background. Yet, a light Higgs pair (Mh +MA < ∼ 70 GeV) may still be accomodated [2]. In this Letter, we concentrate on the scenario wherein either h or A is very light [8]. While, for a light h, this clearly warrants that α ≃ β, it is not necessary if only A is light and Mh > mZ −MA. However, in order to reduce the number of parameters and thus simplify the analysis, we shall not only impose this constraint, but rather promote it to an exact equality. Since we propose to use charged lepton decay modes as our signal, we further restrict ourselves to the scenario with a large tanβ (∼20). Note that, for Mh,A < 10 GeV, this parameter is not Note that there are no tree level ZZA or WWA vertices in this theory.